It has been proposed to utilize thin film resonators, such as stacked crystal filters, as the frequency selective element along with active devices in high frequency oscillator circuits. For example, in commonly assigned Thompson et.al. U.S. application Ser. No. 358,624, now U.S. Pat. No. 4,988,957, there is disclosed a thin film resonator in the form of a stacked crystal filter connected as the feedback element in an amplifier circuit for oscillation at high frequencies. The exemplary embodiment disclosed in the patent application was a microstrip implementation, although the desirability of cointegration was noted.
One of the difficulties involved in cointegrating the thin film resonator and active devices in a monolithic high frequency oscillator is the fact that the processes for producing the active devices and thin film resonator are not in all respects compatible. In order to achieve a high quality high frequency oscillator, the characteristics of both the active devices and the thin film resonator must be optimized, and the processing techniques for forming those respective devices, to the extent they are incompatible, prevent the optimization of the characteristics of both. Certain incompatibilities also have a significant negative impact on yield.
Taken separately, the technology for fabricating high frequency (greater than 2 GHz) junction or trench-isolated buried junction transistors (BJT's) is well known. Various techniques have also been developed and published for forming thin film resonators, although those techniques have certain shortcomings.
The basic thin film resonator technology uses DC magnetron sputtered highly-oriented thin films of dielectric material, preferably aluminum nitride (AlN) or zinc oxide (ZnO). The dielectric film is deposited between a pair of conductive electrodes, typically thin film aluminum electrodes, and the electrodes serve not only as electrical interconnections, but also acoustic reflecting surfaces for guiding and trapping the acoustic energy in the dielectric thin film. The acoustic cavity for the resonator is defined by the piezoelectric-silicon composite membrane structure. That membrane should be of low mass for high frequency operation, and that, in turn usually requires the removal of substrate material underlying the membrane portion of the thin film resonator. It has been typical to accomplish that by first forming a highly doped p+ region near the top surface of the semiconductor substrate, then etching an aperture from the bottom surface terminating at the p+ layer, which functions as an etchant stop. The thin film resonator is then formed on the p+ membrane. After formation of the thin film resonator, a selective etching process removes the p+ membrane, leaving the resonator suspended.
The etching processes for producing that device are such that it has not been practical to form the semiconductor portions of the active device prior to the final etching of the thin film resonator. Thus, the wafer must be substantially fabricated with respect to the thin film resonator before the process for formation of the cointegrated semiconductor devices can begin. However, the process of producing the p+ membrane and the subsequent etching steps, as well as the steps involved in formation of the thin film resonator, have a tendency to propagate defects into the semiconductor substrate which in turn limit the ability to form reliable semiconductor devices in that substrate. For these as well as a variety of other reasons, such as the incompatibility of the p+ implant with the n-type diffusions needed for the high frequency transistors, cointegration has not been entirely feasible. Indeed, the hybrid nature of the devices is believed to have been conventional rather than the exception because of the incompatibility of the processes forming the two types of devices.